Wireless applications are now increasingly numerous. Such applications (for example of the audio and/or video type) require very high bit rates (of the order of one gigabit per second) working up to distances of about 10 meters.
60 GHz radio transmission systems are particularly well suited to data transmission at very high bit rates within a limited radius. Indeed, since air attenuates millimeter RF waves (following physical laws well-known to those skilled in the art), the range is limited to about 10 meters.
Generally, communications networks based on wireless technology using the millimeter RF waveband, i.e. wavebands around 60 GHz, comprise a plurality of nodes having for example a smart reception antenna and a smart emission antenna. Smart antennas are antennas with positive gain that can be used to obtain sufficient radio range without needing to emit at unauthorized power values.
A smart antenna generally consists of a network of radiating elements distributed in matrix form on a support. This type of antenna enables the use of a technique known as beam forming. In this technique, it is possible to control the radiation pattern of the antenna (when emitting and/or when receiving) as well as the orientation of the direction of the radiation pattern by tuning the amplitude and phase of the radio signals (emitted and/or received) by each radiating element of the antenna.
In a classic 60 GHz wireless communication network, the nodes communicate with one another along direct lines of sight (LOS). This maintains a high level of quality of service.
The 60 GHz radio channel is a particularly random transmission channel. This type of channel is especially sensitive to shadowing caused for example by an individual or object that is in the direct line of sight between two nodes of the network. In such a situation, any communication between these two nodes is impossible.
There are several known techniques for minimizing deterioration related to shadowing.
A first prior art technique consists of the use of secondary communications paths.
The term “secondary communications path” is understood to mean a path obtained by reflection on one or more obstacles (such as for example walls, furniture etc). These secondary communications paths are independent and distinct of the direct lines of sight.
This first prior art technique is especially present in the U.S. Pat. No. 6,498,939. This document describes a 60 GHz radio communications network comprising a server with a smart emission antenna and a client having a smart reception antenna. Thus, it is possible to define several distinct communications paths between the server and the client by tuning the antenna orientation in emission and in reception.
The communication network implements a main radio channel using frequencies around 60 GHz through which the server and the client exchange high bit-rate data and a secondary radio channel through which the server and client exchange low bit-rate data.
The data exchanged on the secondary radio channel are control data elements such as for example antenna orientation data, radio signal quality data etc.
The secondary radio channel enables the client and the server to determine the bit error rate (BER) on the main radio channel for various antenna orientations in emission and in reception.
In this first prior art technique, the antenna orientations chosen for the communications on the main radio channel are emission antenna orientations and reception antenna orientations giving the best bit error rate of the moment. Thus, if the direct line of sight is free (with no shadowing), the choice of orientation of the antennas will correspond to the direct line of sight between the server and the client. By contrast, if the direct line of sight is shadowed by an obstacle, the choice of orientation of the antennas will correspond to a secondary communication network.
This first prior art technique has a certain number of drawbacks.
First of all, this technique calls for the implementation of a main radio channel, a secondary radio channel and a bit error rate measurement mechanism for measuring the bit error rate on the main radio channel. This is particularly complex and costly.
Furthermore, to ensure optimal quality of service, the measurements of bit error rate on the main radio channel should be done constantly. The bit error rate measurement mechanism therefore proves to be costly in terms of computation power and electrical power supply.
Furthermore, the switching to new antenna orientations may temporarily downgrade the quality of service of the application.
A second prior art technique lies in the use of a mesh technique.
In a mesh network, all the nodes of the network communicate with each other by distinct radio paths. This type of network enables the introduction of spatial diversity and an increase in data redundancy. Thus the mesh technique improves the reliability of communications between the different nodes of the network.
The use of the mesh technique in a wireless network makes it possible to multiply the data reception paths for each of the nodes of the network. Thus, each node receives several copies of a same piece of data through different reception paths. The different copies are compared to detect errors and then as the case may be these errors are corrected by means for an appropriate error correction code.
In general, a wireless mesh network implements a TDM (Time Division Multiplexing) type transmission protocol. The TDM protocol consists in sharing the transmission time between the different nodes of the network. More specifically, each node is a sender during a predetermined time slot and is a receiver during all the other time slots. Here below in this document, the term “frame” (or “speech time”) refers to a time interval or time slot during which a given node sends information, and the term “super frame” (or “TDM sequence”) refers to the concatenation of all the frames of a TDM cycle.
Wireless mesh networks are especially well suited to connectivity between the different elements of a home cinema.
Here below, the description is situated in the context of a 7.1 type wireless home cinema system, i.e. an eight-channel audio system.
For example, the 7.1 wireless home cinema system is laid out in a room of a dwelling and comprises an audio-video source terminal, for example a DVD player, a television screen, a wireless surround controller (WSC) node to which the following are connected through a wireless network: first, second, third, fourth, fifth, sixth, seventh and eighth wireless active speakers here below called WAS nodes.
In this example, each WAS node has (or is associated with) an acoustic speaker which broadcasts one audio channel among the following channels FL (for Front Left), FR (for Front Right), C (Centre), SL (Surround Left), SR (Surround Right), RL (Rear Left), RR (Right Rear) and SW (Subwoofer).
A description is now provided of the working of a classic wireless 7.1 home cinema system.
The source terminal sends multi-channel digital audio content to the WSC node through a digital audio-video interface (or purely audio interface) which may be compliant for example with the SPDIF, IEEE-1394 or HDMI standards.
The WSC node receives the multi-channel digital audio content and then decodes and decompresses the audio data of the different audio channels (C, FL, FR, SL, SR, RL and RR.
Then, the WSC node inserts these audio data elements into radio packets. Finally, the WSC node transmits the radio packets to the WAS nodes through the 60 GHz radio channel.
After reception of the radio packets, each WAS node extracts the radio data corresponding to the audio channel assigned to it (C, FL, FR, SL, SR, RL or RR) from the radio packets. Each WAS node then makes a digital/analog conversion on the extracted audio data so as to obtain an audio signal. Finally, each WAS node amplifies the radio signal obtained and converts it into an acoustic signal so as to restore it through its acoustic speaker.
It may be recalled that in a wireless mesh network each node of the network behaves like a sender node during a predetermined frame of the super frame and like a receiver node during all the other frames of the super frame. When a node behaves like a sender node, its smart antenna is controlled to form a wide radiation pattern so it can reach a maximum number of nodes (WAS and WSC) of the network. By contrast, when a node behaves like a receiver node, its antenna is controlled to form a narrow, orientable radiation pattern so as to obtain high antenna gain and thus reach the distances required by the application.
Each receiver node orients its antenna at an angle adapted to the reception of audio data sent out by the current sender node. It must be noted that at each new frame (in other words at each change of a sender node), each receiver node must reorient its antenna at a new angle adapted to the reception of the audio data sent by the new sender node. To this end, each node of the wireless mesh network manages a table in which the angles of orientation of its reception antenna are stored. Each antenna orientation angle corresponds to a direct line of sight to communicate with another node of the network. This table may for example be initialized when the home cinema system is first put into service at the user's premises or again upon a request by this user through the home/machine interface of the system.
The mesh technique requires a rigorous spatial and temporal synchronization. Indeed, each node of the network must orient its antenna in the right direction and at the right time.
The mesh technique is well suited to wireless networks having a large number of nodes. However, for networks having a limited number of nodes, it is not optimal.
Indeed, for a 2.1 type home cinema system having only four nodes, the number of distinct radio paths is small. Thus, in certain configurations, when a shadowing comes into play on one of the radio paths, it may happen that certain nodes do not receive a sufficient number of copies of the same data to detect and correct the errors reliably.
Furthermore, it is noted that certain applications can dictate special constraints on the position of the nodes in the wireless mesh network. This is the case for example for the home cinema system described here above in which the position of the WAS nodes is dictated by the audio channel rendered by each of the WAS nodes. Thus, a node may be positioned in the network in such a way that it communicates with other nodes by using antenna orientation angle values that are close to each other. It may be recalled that each antenna orientation angle corresponds to a direct line of sight. Consequently, another major drawback of classic mesh technique lies in the fact that a same obstacle can disturb several direct lines of sight of a node (if this node uses antenna orientation angles that are close to each other), thus harming the overall quality of service of the network.